Recently, with the development of the electronics industry, the use of a polymer resin for optical use has rapidly increased. For example, the demand for various polymer materials for optical use, such as an optical disc substrate used as a storage medium, an optical lens, an optical fiber used for optical communication, a fresnel lens for a projection screen, a prism sheet used in a liquid crystal display, a protective film for a polarizing plate, a compensation film, etc., has been rapidly increased.
Such polymer resins for optical use are usually required to be optically isotropic, but a polymer resin of a final product may be anisotropic due to its own structure or stress during the process. Also, an optical resin, when inserted in a multi-layered structure, may have optical anisotropy by thermal expansion/contraction between respective layers.
When a film, lens, etc. with such an optical anisotropy exists in an optical path, a property of the phase may be changed, thereby adversely affecting signal reading. Therefore, it is preferable to use an optical member including an optical resin of which anisotropy is reduced as much as possible.
On the other hand, optical birefringence may be artificially given. For example, in the case of a liquid crystal display, the contrast at an inclined viewing angle may be degraded due to optical birefringence caused by the liquid crystal and a polarizing film. In order to solve this problem, a polymer resin having optical anisotropy with a certain direction may be used to compensate the birefringence caused by the liquid crystal and the polarizing film, thereby improving a viewing angle. In the case of a polymer resin for optical use as mentioned above, the optical characteristic is required to be controlled in a required direction.
As a conventional technology of controlling the optical characteristic of a polymer, a birefringence control method through adjustment of the structure of a polymer resin, such as Japanese Laid-Open Patent Nos. 1990-129211 and 2000-044614, has been known. However, the method has a problem in that the composition of a monomer of a copolymer is required to be changed to control the birefringence, and the change in the composition of the monomer may change other physical properties of the polymer resin.
Also, as a birefringence control method of a polymer resin, methods of adding an anisotropic material have been known. Japanese Laid-Open Patent No. 2004-035347 disclosed a method of introducing a needle-shaped inorganic particle as an anisotropic material to adjust an optical characteristic. However, in the case of an inorganic fine particle, it is difficult to uniformly disperse the particle in a solvent due to low solubility in the solvent and high density, and there is a possibility that the particle is subject to aggregation due to low stability, thereby reducing the transparency of the polymer resin. Also, Japanese Laid-Open Patent No. Hei 8-110402 disclosed a method of adjusting the optical characteristic of a polymer resin by adding a low-molecular organic compound. In general, compounds with high anisotropy have disadvantages, such as high crystallinity, low solubility in a solvent, and low compatibility with the polymer resin. Thus, the compounds, when mixed, may be precipitated by crystallization, and may be not dissolved in a solvent. However, there is no specific solution for such problems.